1. Field
Apparatuses and methods consistent with exemplary embodiments relate to compressor control, and more particularly, to compressor control that stably operates a compressor by preventing occurrence of a surge.
2. Description of the Related Art
A compressor compressing a fluid may be used in a fluid control system controlling a liquid or a gaseous fluid. A compressor may be designed to operate at high efficiency with respect to a possibly wide range of discharge a pressure and a flow, and not only the efficiency of the compressor but also an operating region thereof may act as important system performance parameters.
For example, in the case of a centrifugal turbo-compressor, if a pressure at a rear end of the compressor exceeds acceptable performance of the entire fluid control system when the compressor receives and compresses a gas, the compressor may no longer compress the gas and thus a periodic fluid backflow phenomenon may occur in the compressor, which may be referred to as a surge.
An impeller of a compressor may have a fixed pressure ratio at a constant speed and a constant density, and the fixed pressure ratio may be referred to as a specific pressure ratio of the compressor. A surge may occur when the compressor operates above its specific pressure ratio. A high-power compressor may be implemented by obtaining a high pressure ratio by increasing the number of stages of a compressor; however, a surge phenomenon may occur more frequently in a multi-stage compressor having many stages.
Most surge phenomena may occur shortly at a speed higher than reaction speed of instruments. In this case, when a fluid mechanical system fails to avoid a surge phenomenon and thus a surge phenomenon occurs therein, a fluid may periodically flow back, and thus, a pressure and a flow thereof may fluctuate. This fluctuation may generate a mechanical vibration and damage accessory elements such as bearings and impellers.
In this manner, since a surge phenomenon may degrade the performance of a compressor and shorten the life of a compressor, a function of preventing a surge phenomenon in operating a compressor (i.e., an anti-surge function) may be important in a compressor control system controlling a turbo-compressor. The anti-surge function may be technology for restoring a fluid system to normal or preventing occurrence of a surge therein by measuring and analyzing various characteristics of the fluid system caused by a surge phenomenon.
When an anti-surge valve (ASV) is used to implement the anti-surge function, the resistance of the fluid system may be reduced to prevent occurrence of a surge phenomenon therein.
Also, in addition to the ASV, the compressor control system may include an inlet guide vane (IGV) installed at an inlet of the compressor to control an operating region of the compressor.
FIG. 1 is a graph illustrating a performance map of a compressor.
In FIG. 1, the vertical axis represents pressures or pressure ratios, and the horizontal axis represents parameters such as flows or motor currents representing flows.
As for surge control, a surge control range is set to have a margin of about 7% to about 10% from a surge range in consideration of a measurement error in a sensor, a temperature change in an intercooler, or a transient response in compressor control, and control is performed to move an operating point away from the surge range by adjusting an IGV or an ASV when the operating point reaches the surge control range.
Also, in order to adjust a pressure of a fluid system to a set pressure, IGV control is performed as follows. That is, when a current outlet pressure of the compressor is lower than a set pressure, the IGV is opened by a pressure gap or difference to adjust the pressure of the fluid system to the set pressure; and when the current outlet pressure of the compressor is higher than the set pressure, the IGV is closed by the pressure gap. By this IGV control, the current outlet pressure of the compressor may follow the set pressure.
A centrifugal turbo-compressor operates in a range of a turndown that is a flow variation from a design point of a performance curve to the surge control range before occurrence of a surge phenomenon.
When a flow decreases rapidly in the compressor operating in the turndown, since the outlet pressure increases and thus the operating point enters a region of the surge control range, both the IGV and the ASV operate. When the IGV is further closed, since an intake flow of the compressor decreases, a flow of the entire compressor decreases and thus the operating point moves toward the surge range. In this case, when the operating point approaches the region of the surge control range in a state where the pressure is higher than the set pressure, since an IGV pressure controller controls the IGV in a direction of further closing the IGV, a possibility of occurrence of a surge phenomenon increases.
FIG. 2 is a graph illustrating a coupling phenomenon occurring in an IGV and an ASV in a compressor of FIG. 1.
For example, if the compressor is controlled at an operating point represented by “A” in FIG. 2, a direction for controlling the IGV and a direction for controlling the ASV may collide with each other. That is, in order to reduce the pressure thereof, the IGV has to be controlled in a direction of reducing (closing) an opening degree of the IGV (an aperture of the IGV). When the control is performed to reduce the aperture of the IGV, since the flow and pressure thereof decrease, a control point of the IGV moves in a direction toward the bottom left side in FIG. 2.
However, in order to increase a flow to prevent a surge phenomenon, the ASV has to be controlled in a direction of opening an aperture of the ASV. When the control is performed to open the aperture of the ASV, since the flow thereof increases and the pressure thereof decreases, a control point of the ASV moves in a direction toward the bottom right side in FIG. 2. In this manner, since a collision occurs between the control operations of the IGV and the ASV, and therefore, a pressure hunting phenomenon occurs and thus an unstable flow is repeated, the operation of the compressor becomes unstable.
This coupling phenomenon occurs because the discharge pressure of the compressor is controlled by the operation of the IGV but the flow thereof is influenced by the operation of the IGV, and the control based on the flow of the compressor may be performed by the operation of the ASV but the pressure thereof is influenced by the operation of the ASV. Thus, since two valves of the IGV and the ASV interfere with each other in a surge region, it becomes difficult to control the entire system of the compressor.
In order to avoid a collision problem between the control operations of the ASV and the IGV as described above, the control gains of a proportional integral derivative (PID) controller for controlling the IGV and the ASV may be set to be different from each other, and the control gain of any one of the IGV and the ASV may be set to be dominant, thereby reducing the collision in the surge region. However, this gain control method may be complex and difficult in terms of controller tuning, and may fail to perfectly cope with the coupling phenomenon.
For example, when the IGV gain is set to be greater than the ASV gain, since the pressure thereof may be stabilized but the operating point thereof may enter the surge region, a possibility of causing a surge phenomenon may increase or it may be difficult to cope with a rapid change in the flow consumption at the rear end of the compressor.
Also, when the ASV gain is set to be greater than the IGV gain, since the ASV may be rapidly opened when the operating point enters the surge region, the width of a pressure drop may increase. In this case, it may be possible to cope with a rapid flow consumption change, but the operation of the compressor may become unstable because the width of a pressure drop may increase.